Chapter 24: The Respiratory System
The respiratory system includes those structures beginning at the nose and extending to the alveoli of the lungs where gas exchange occurs.
Functions of the respiratory system:
Respiratory epithelium is pseudostratified ciliated columnar epithelium with numerous goblet cells which produce mucus. Exceptions are the alveoli where gas exchange occurs which is a simple squamous epithelium and the oropharynx which is stratified squamous.
In the nasal cavity the cilia move debris and inhaled microorganisms to the pharynx where they are swallowed and destroyed. In portions below the pharynx the cilia move the same in a cephalad direction to the pharynx. Expectoration is the process of coughing something up from the lungs or lower respiratory tract. Filtration, warming and humidification of inspired air occur throughout the conducting portion of the respiratory system.
The Upper respiratory System
The Nose and Nasal Cavity: studied in detail in skeletal anatomy and special senses.
Nasopharynx: that part posterior to the nasal cavity, lined with respiratory epithelium (pseudostratified ciliated columnar epithelium).
Oropharynx: extends from soft palate to hyoid bone. This cavity is shared with the digestive system and is lined with stratified squamous epithelium.
Laryngopharynx (hypopharynx): hyoid bone to entrance of the esophagus, lined with stratified squamous epithelium.
The Lower Respiratory System
The larynx: Extends from the top of the thyroid cartilage to the top of the trachea. It surrounds the glottis which is the opening between the vocal cords.
Three cartilages of the body of the larynx: thyroid, cricoid, and epiglottis. The epiglottis closes over the glottis during swallowing to prevent aspiration of food.
The paired arytenoid cartilages sit atop the cricoid cartilage and is the sight of attachment of the vocal cords which extend from the thyroid cartilage.
Intrinsic and extrinsic ligaments bind the cartilages together to form the larynx.
Air passing through the vocal cords is responsible for sound. The width, tension and diameter of the glottis are responsible for the pitch.
Intrinsic laryngeal musculature is responsible for the tension of the cords and opening and closing of the glottis.
The Trachea: Extends from the larynx into the chest and then bifurcates into the right and left mainstem bronchi at the carina. There are 15 to 20 C-shaped tracheal cartilages which are incomplete posteriorly allowing “give” from the trachea during a swallow as the bolus passes through the esophagus. Annular ligaments bind the tracheal cartilage together.
The right mainstem bronchus has a more direct downward slope than the right which means that most aspirated foreign bodies end up there. The same C-shaped cartilaginous rings found in the trachea are present in the mainstem bronchi. The bronchus enters the lung at the hilum or root before dividing further. On the right, the first division generates the upper lobar bronchus and the bronchus intermedius, which then divides into the middle and lower lobar bronchi. On the left the first division generates the upper and lower lobar bronchi. The bronchi continue to divide into ever smaller branches until reaching terminal bronchioles, then respiratory bronchioles and alveoli in lobules. The terminal bronchioles have no cartilage but do have smooth muscle in the walls which can respond to autonomic stimulation and dilate (bronchodilation) or constrict (bronchoconstriction). The bronchi run in connective tissue with the pulmonary vasculature (which is why it’s hard to stick a needle in a lung without the patient coughing up blood, hemoptysis).
The right and left lungs occupy the right and left pleural cavities. Each lung has a somewhat pointed apex at the cephalad aspect and a flatter base inferiorly where it interfaces with the diaphragm. The right lung has three lobes; upper, middle and lower, separated by the major and minor fissures. The left lung has two, an upper and lower separated by the major fissure. The lobes are further divided into bronchopulmonary segments.
The alveolus is where the exchange of gasses occurs (respiration) and has an associated extensive network of capillaries.
Alveolar epithelium is primarily simple squamous. Some alveolar cells produce surfactant which keeps the alveolus from collapsing upon exhalation. Macrophages are also present in the alveoli.
Blood supply to and from the lungs is via pulmonary arteries and pulmonary veins.
Pleural Cavities and Pleural Membranes:
The paired pleural cavities are separated by the mediastinum. The lungs are lined by visceral pleura; the pleural cavity is lined by parietal pleura. There is a small amount of pleural fluid between these two layers of pleura which acts as a lubricant and helps keep the lungs expanded. When air insinuates itself between the pleural surfaces, breaking this seal of pleural fluid a pneumothorax occurs. Inflammation of the pleura results in pleurisy and pleuritic chest pain which is typically most painful on deep inspiration.
Respiratory Muscles and Pulmonary Ventilation: Most important is the diaphragm with the intercostals running second.
Respiratory Centers of the Brain: Three nuclei located in the pons and medulla oblongata.
